This invention relates generally to the production of octene isomers by the oligomerization of butene. Specifically, the invention relates to the oligomerization of butene to high selectivities of octene isomers and, specifically, to 2,4,4-trimethylpentene.
Processes for the oligomerization of lighter olefins to produce C8 oligomers are known. Oligomerization processes have been long employed to produce good quality motor fuel from butene. Such oligomerization processes are also referred to as catalytic condensation and polymerization with the resulting motor fuel often referred to as polymer gasoline. Methods have always been sought to improve the octane number of the gasoline boiling range oligomerization products. In addition, the oligomerization process is also susceptible to catalyst fouling from the condensation of heavy oligomers into coke that covers the catalyst.
Another process that has met the continuing demand for the conversion of light hydrocarbons into high octane motor fuels was the alkylation of isobutane with propylene, butenes and amylenes using a hydrofluoric acid (HF) catalyst, commonly referred to as HF alkylation. The HF process has provided a highly successful method for the production of high octane motor fuels.
A number of arrangements are known for using oligomerization in combination with other processes such as saturation and dehydrogenation as substitutes for acid catalyzed isomerization alkylation. Patents disclosing the dehydrogenation of light paraffin stream with oligomerization of the dehydrogenation effluent include U.S. Pat. No. 4,393,259 B1, U.S. Pat. No. 5,049,360 B1, U.S. Pat. No. 4,749,820 B1, U.S. Pat. No. 4,304,948 B1 and U.S. Pat. No. 2,526,966 B1.
Trimethylpentenes are the preferred product in the production of gasoline. High selectivities to trimethylpentenes, and specifically to 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene, are desired because they can be hydrogenated to 2,2,4-trimethylpentane which has a very high research and motor octane numbers.
Isomers of 2,4,4-trimethylpentene typically include 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene but will hereinafter be collectively referred to as 2,4,4-2-trimethylpentene. Among the trimethylpentanes, 2,2,4-trimethylpentane is desired when a high vapor pressure gasoline blending stock is desired because it has a relatively low boiling point and a relatively high vapor pressure.
U.S. Pat. No. 5,877,372 B1 endeavors to oligomerize to diisobutene by dimerization of pure isobutene. Diisobutene is typically 2,4,4-trimethylpentene, but it tends to isomerize to 2,3,4-trimethylpentene. Affordable feedstocks for butene oligomerization processes are typically provided by one of the effluent streams from a fluidized catalytic cracking unit which usually includes a mixture of isobutene, n-butene and butane. Hence, dimerization of pure isobutene is not typically feasible.
Efforts in the prior art to increase the selectivity to 2,4,4-trimethylpentene from butene mixtures typically involve attempting to encourage the dimerization of isobutene and suppress both the dimerization of n-butene and the co-dimerization of isobutene and n-butene. U.S. Pat. No. 4,469,911 B1 discloses oligomerizing isobutene and n-butene together over a resin catalyst at lower temperatures. The lower temperatures are reported to favor selective dimerization of isobutene with itself to produce isobutene dimer or diisobutene rather than the codimerization of isobutene with n-butene to produce codimers or subsequent oligomerization to produce trimers. Although this process is reported to produce a selectivity to 2,4,4-trimethylpentene as high as 86.2 liquid volume percent, the attendant selectivity to dodecene was as high as 12.2 liquid volume percent. Dodecene in the product lowers octane numbers and lowers vapor pressure.
Other patents disclose oligomerizing a mixture of n-butenes and isobutenes under conditions that encourage isobutene dimerization and discourage n-butene dimerization. In U.S. Pat. No. 4,197,185 B1, U.S. Pat. No. 4,244,806 B1 and U.S. Pat. No. 4,324,646 B1, a cut including isobutene, n-butene and butane is oligomerized over an alumina based catalyst such that the isobutene has at least a 90% conversion and the n-butene has lower than a 16% conversion. This method capitalizes on the slower rate of n-butene reactions. U.S. Pat. No. 3,832,418 B1 also discloses a selective dimerization process in which a mixture of n-butenes and isobutenes oligomerize over a catalyst comprising presulfided nickel fluorine on a silica-alumina support with over 80% conversion of isobutene and less than 5% conversion of n-butene. U.S. Pat. No. 5,994,601 B1 discloses oligomerizing a mixture of n-butenes and isobutenes while endeavoring to separate dimers of the n-butenes from dimers of isobutene.
Contrarily, other patents focus on n-butene dimerization. U.S. Pat. No. 4,225,743 B1 discloses codimerizing isobutene with n-butene to form methylheptenes and dimethylhexenes and suppress the formation of 2,4,4-trimethylpentene by using a specific nickel catalyst solution and an organo-aluminum catalyst. U.S. Pat. No. 4,463,211 B1 discloses that dimerization of n-butenes in the presence of minimal isobutenes over a cation exchange resin yields primarily dimethylhexenes. Pimethylhexene reduces the octane number of gasoline.
The indirect alkylation process described in U.S. Pat. No. 6,080,903 B1, U.S. Pat. No. 5,990,367 B1 and U.S. Pat. No. 5,895,830 B1 dimerizes mixtures of n-butene and isobutene over a solid phosphoric acid (SPA) catalyst in the presence of a higher paraffin diluent such as cyclohexane or octane. The presence of the paraffin diluent is believed to promote the oligomerization in the liquid phase to yield predominantly dimerized butene oligomers such as C8 olefins. The liquid phase washes deactivating components from the catalyst to prolong catalyst life. The higher aliphatic olefins can be saturated to provide high octane fuel. The process gives high butene conversion with octene selectivities as high as 87.2 wt-% and selectivities to trimer products as low as 11.7 wt-%. These patents recommend that operating temperatures in a narrow range of 300xc2x0 to 400xc2x0 F. (149xc2x0 to 204xc2x0 C.) increase the selectivity of C8 olefins.
Even in the context of indirect alkylation, mechanistic theory predicts that in a reaction mixture of isobutene and n-butene in the presence of a SPA catalyst, the isobutene will dimerize with itself to produce 2,4,4-trimethylpentene and isobutene will co-dimerize with n-butene to produce 2,2,3-trimethylpentene. Moreover, the desired 2,4,4-trimethylpentene also has a tendency to shift to 2,3,4-trimethylpentene.
Hence, it is an object of the present invention to run an oligomerization of butene in the presence of a catalyst so as to obtain a desired selectivity to 2,4,4-trimethylpentene from an oligomerization of butenes. It is a further object of this invention to oligomerize a mixture of n-butene and isobutene to obtain a high selectivity to 2,4,4-trimethylpentene. It is a still further object of the present invention to minimize the production of dodecene.
It has been surprisingly found that very high yields of octenes and, specifically, 2,4,4-trimethylpentene, with attendant low selectivity to dodecene are produced from the dimerization of isobutene and/or mixtures of isobutene and n-butene when diluted with a paraffinic diluent. We have surprisingly found that higher conversion of n-butene with isobutene does not diminish the selectivity to 2,4,4-trimethylpentene. In light of earlier belief, we were surprised to find that high selectivity to octene can be achieved at lower temperatures, such as below 250xc2x0 F. (121xc2x0 C.). Moreover, we found that by diluting the oligomerization conditions with more paraffinic diluent, octene selectivity exceeds 98% and 2,4,4-trimethylpentene selectivity exceeds 77 wt-%, while producing less than 0.9 wt-% dodecene.
In one embodiment, the present invention relates to a process for oligomerizing isobutene and n-butene to a product comprising a high selectivity to 2,4,4-trimethylpentene. The process comprises passing an olefinic feed comprising isobutene and n-butene to an oligomerization zone and contacting the olefinic feed with an oligomerization catalyst at oligomerization conditions including a reaction temperature of 250xc2x0 F. (121xc2x0 C.) or less. A saturate stream comprising paraffins having a carbon number of at least 6 is passed into the oligomerization zone with the olefinic feed and the catalyst at a predetermined weight ratio of the saturate stream to the olefinic feed. An effluent stream including product exhibiting a greater selectivity to 2,4,4-trimethylpentene than an effluent stream from a process with substantially the same conditions but with a smaller weight ratio of saturate stream to olefinic feed is recovered from the process.
In another embodiment, the present invention relates to an oligomerization process with a high selectivity to 2,4,4-trimethylpentene. The process comprises passing an olefinic feed comprising n-butene and isobutene to an oligomerization zone and contacting the olefinic feed with a solid phosphoric acid catalyst at oligomerization conditions including a reaction inlet temperature of no greater than 250xc2x0 F. (121xc2x0 C.). A saturate stream comprising paraffins having a carbon number of at least 6 is passed into the oligomerization zone with the olefinic feed and the catalyst. An effluent stream including product exhibiting a selectivity to 2,4,4-trimethylpentene of at least 40 wt-% is recovered from the process.
In a further embodiment, the present invention relates to an oligomerization process with a high selectivity to octene and a low selectivity to dodecene in an effluent stream. The process comprises passing an olefinic feed comprising butene to an oligomerization zone and contacting the olefinic feed with an oligomerization catalyst at oligomerization conditions including a reaction temperature of 250xc2x0 F. (121xc2x0 C.) or less. A saturate stream comprising paraffins having a carbon number of at least 6 is passed into the oligomerization zone with the olefinic feed and the catalyst. An effluent stream comprising the paraffins and a product exhibiting a selectivity to octene of at least 80 wt-% and a selectivity to dodecene of 11 wt-% or less is recovered from the process.
Other objects, embodiments and details of this invention will be provided in the following detailed disclosure of the invention.